Intaglio printing is typically used for the manufacture of documents of value such as bank notes for which counterfeiting and fraudulent actions are a cause of concern. According to this printing method, a substrate such as a high grade paper is pressed against an engraved and inked plate, at very high pressure, so as to cause the substrate to deform into ink-bearing engraved recesses of the plate and thereby produce an inked impression on the substrate, the appearance of which is defined by the engravings and is difficult to replicate without using an intaglio printing press.
However, with the advent of improved technologies for photocopiers an increased risk of counterfeiting of intaglio-printed documents, using photographic techniques, has developed. These new technologies can be capable of reproducing intaglio-printed documents with sufficient quality to be able to avoiding detection by lay persons relying only on a visual inspection of the reproduced document. Consequently, there is a need for means to enable authentication of such documents through a simple visual inspection of the document (i.e. without the use of special knowledge or experience, finding screens, special lights or similar tools).
Certain specific printed security devices are known for facilitating such authentication but, by reason of their manner of use and limited visual effects, these known devices are primarily directed to trained or knowledgeable security personnel rather than for the use of lay persons. For example, U.S. Pat. No. 4,033,059 to Hutton et al describes an intaglio printing of two images, one being a background image and the other a latent image, using differing orientation or depths for the intaglio imprints such that the latent and background images are not discernible and appear as a single image when viewed from a normal viewing angle roughly perpendicular to the document but when viewed from the side (i.e. at a narrow angle to the plane of the document) the latent image stands out as an independent image separate and discernible from the background. According to that security device it is necessary to decrease the viewing angle (that is the angle to the plane of the document) in order to establish and then improve the visibility of the latent image. Similarly, U.S. Pat. No. 5,199,744 to Shenton describes a security device comprising a substrate which is embossed with a transitory image on one area of a substrate, the transitory image being visible at a specific angle only and not at other angles, and a linear area around the image which is visible at all angles, such that when the device is viewed at angles other than the specific viewing angle a uniform image is seen and the transitory image (being either a latent or transient image) is not discernible (this being discernible as a separate image only when viewed at the specific viewing angle). Thus, for both of these known security devices their effective use is directed to persons having knowledge of the applicable viewing angles.
Another example is a security device described in Canadian Patent No. 1,172,282 to Merry whereby a first line deflection pattern, defining a first image which is detectable by using a finding screen there over, overlays a second line deflection pattern, defining a second image which is detectable by using a finding screen there over, the two patterns being printed at different angles of orientation such that each image is separately detectable using a finding screen oriented at such different angles. Similarly, Canadian Patent No. 2,071,795 to Merry et al describes a security device comprising an array of micro-characters in which a group of the characters is phase-shifted relative to the others to collectively define an image, whereby the characters appear uniform when viewed by the naked eye and the image is not disvernible but with the aid of a finding screen the image becomes visible. Effective use of these security devices by persons requires such persons to be knowledgeable with respect to the such use and also requires a specific finding screen.
There is a need, therefore, for a security device which is useable without prior knowledge by the user. Further, there is a need for a security device which can be applied to a document using conventional printing and is relatively simple and avoids high manufacturing costs. Still further, there is a need for a security device which provides a distinct visual effect as compared with those created by the known latent and transient images described in the foregoing patent references.